The goal of this research proposal is to characterize dopaminergic receptors invertebrate retinas. Dopamine is the predominant catecholamine of the retina, and it has been postulated to mediate a variety of cellular responses to light onset or offset. Dopamine interacts with two basic types of receptor that have been classified as Dl and D2. These receptors are coupled to stimulation (Dl) and inhibition (D2) of adenylate cyclase and possibly to other signaling mechanisms. Each of these receptor types is present in retina and stimulation of each has been suggested to have specific physiological effects. This research program will use morphological, biochemical, and molecular approaches to study the function and regulation of retinal dopaminergic receptors under both normal and pathological conditions. To accomplish this goal, three specific aims are proposed: 1) Dopaminergic receptors and their MRNA will be localized by immunocytochemistry and in situ hybridization. Knowledge of the distribution of these receptors will prove useful in assessing the role of dopamine as a diffusible neuromodulator. 2) The mechanisms modulating dopaminergic receptor sensitivity and MRNA levels in response to signals from diurnal and circadian stimuli will be analyzed. Receptor sensitivity will also be assessed following MPPI+ administration. This chemical agent depletes levels of endogenous dopamine and induces symptoms mimicking Parkinson's disease. MPP+ -treated retinas thus provide an experimental model for examining the regulation of dopaminergic receptors in clinical disorders of the CNS. 3) The existence of dopaminergic receptor subtypes will be investigated by screening human and goldfish retinal CDNA libraries with a probe based on a recently obtained D2 dopaminergic receptor clone. These studies may lead to the discovery of a Dl dopaminergic receptor clone and to the identification of subtypes of D2 dopaminergic receptors. If so, these newly found cDNAs will be pharmacologically and physiologically characterized by in vitro expression. This project will help to clarify the role of dopamine and its receptors in mediating basic physiological processes in vertebrate retinas. Moreover, it presents an excellent opportunity to utilize the retina as a model system for investigating receptor regulation and receptor subtypes. These results may have direct clinical applications in the retina and elsewhere in the CNS.